Mobile station positioning has become increasingly important, not only for supporting enhanced emergency calling (e.g., E-911 in the United States), but also for supporting commercial location-based services. Although several technologies for determining the location of wireless mobile stations have been deployed, a technique called Observed Time-Difference Of Arrival (OTDOA) is widely used in modern cellular telecommunications networks.
With the OTDOA technique, a mobile station's location can be determined based on measurements of the following parameters: (1) time-difference-of-arrival (TDOA) measurements of downlink radio signals received at a mobile station from several base stations; (2) actual real time differences (RTDs) between the transmissions of pairs of base stations, at the time when the TDOA measurements are made; and (3) geographical positions (e.g., latitude and longitude) of the several base stations. Measurements using downlink signals from at three different base stations are required. The accuracy of each of these measurements contributes to the overall accuracy of the position estimate. However, more TDOA measurements bring better accuracy.
There are several approaches to determining the real time difference for a pair of base stations. One technique involves transmissions from base stations that are synchronized to one another. In this case the RTD for any given pair of base stations is a known constant value that may be stored in a database and used by the positioning function when making a position estimate based on TDOA measurements for that base station pair. For optimal accuracy, the synchronization should be done to a level of accuracy on the order of tens of nanoseconds, as only ten nanoseconds of uncertainty contributes 3 meters of error to the position estimate. Drift and jitter in the synchronization timing must also be well controlled, as these also contribute to uncertainty in the position estimate. Currently, synchronization to this level of accuracy is currently only readily available through satellite-based time transfer techniques. Base stations in systems employing a time-division duplexing (TDD) operating mode are often synchronized.
Alternatively, base stations may be left to run “free,” within some constraint on the maximum frequency error allowed in the system. In this scenario, the RTD will change over time, although usually slowly, given tight frequency accuracy specifications for the controlling reference clocks. The rate of change will depend on the frequency differences between the reference clocks for a given pair of base stations, as well as on the jitter associated with each clock.
The OTDOA positioning technique may be applied in at least two modes: UE-assisted OTDOA and UE-based OTDOA. (“UE”, or “User Equipment”, is a term used in standards promulgated by the 3rd Generation Partnership Project to refer to end-user wireless communication devices. As used herein, the terms “UE,” “mobile station,” and “mobile terminal” are equivalent, and are intended to generally refer to an end-user wireless communication device, whether portable or fixed, or whether self-contained or built into another device such as a personal computer or an automobile. These terms are thus intended to encompass, without limitation, machine-to-machine devices as well as handheld mobile phones.) These two modes differ in where the actual position calculation is carried out. In the UE-assisted mode, the mobile station measures the TDOA of several cells and sends the measurement results to the network, where a positioning node (e.g., a location server) carries out the position calculation. In the UE-based mode, on the other hand, the mobile station makes the measurements and carries out the position calculation as well. To perform UE-based positioning, the mobile station clearly requires additional information, such as the position of the measured base stations and the timing relationships among the base stations.
OTDOA has been standardized by 3GPP for GSM/EDGE Radio Access Networks (GERAN) as well as for UMTS Radio Access Network (UTRAN). (In the former specification, the technique is referred to as Enhanced Observed-Time-Difference, or E-OTD.) Standardization in 3GPP of positioning techniques for Evolved UTRAN (E-UTRAN) is still ongoing, but OTDOA has already been widely accepted as a very important positioning method In fact, some U.S. operators have begun planning for OTDOA deployment in Long Term Evolution (LTE) networks in about 2010 or 2011. Moreover, it is also very clear that OTDOA-related protocols in E-UTRAN will soon be adopted by the Open Mobile Alliance (OMA) as a basis for so-called User Plane positioning. As a result, OTDOA-based positioning techniques are continuing to grow in importance.